In recent years, there is demand for display devices used in the area of industrial appliances (IA) to be safe, economical, and environmentally friendly. Based on this, consideration has been given to using light-emitting diodes (LEDs) as the light source for the backlight in liquid crystal displays (LCDs) in order to improve lifespan compared to conventional devices.
In such applications, a plurality of LEDs are provided in a line with a plurality of such lines being provided, and modularization has taken place in order to be able to simultaneously drive many LEDs.
FIG. 6(a) shows a configuration of a general LED driver circuit 100. As shown in FIG. 6(a), the LED driver circuit 100 includes a light-emitting module 110. The light-emitting module 110 includes light-element lines L101 and L102 in which a plurality of light-emitting elements are provided in a line, and the respective light-element lines are connected to each other in parallel.
The LED driver circuit 100 includes a general use LED driving integrated circuit (IC) 101, a semiconductor switch 102, a voltage source 103, an inductor 104, a diode 105, and a capacitor 106.
The general use LED driving IC 101 is connected to other components through a current input terminal ITi and a control output terminal OT, thereby forming the LED driver circuit 100. By this configuration, the general use LED driving IC 101 controls the value of a current Iall flowing in the LED driver circuit 100 so as to be a fixed value (40 mA). By performing such control, even if the value of the current Iall fluctuates, the value of the current Iall can be kept at the constant rate by the general use LED driving IC 101 opening and closing the semiconductor switch 102. In other words, the LED driver circuit 100 controls a few light-element lines at a time on the cathode side of the diode 105 by controlling the sum of the currents flowing through the light-element lines.
The number of light-element lines, the arrangement, the number of light-emitting elements (LEDs) included in each light-element line, and the like can be changed according to the specifications of the general use LED driving IC (the allowable current value, the maximum number of lines, etc., for example).
However, the LED driver circuit 100 has the following problems.
FIG. 6(b) is a drawing showing the state of the light-emitting module 110 when the light-element line L101 is open. Here, “open” refers to the fact that there is a disconnection in a portion of the light-element line. As shown in FIG. 6(b) if the light-element line L101 is open, no current flows through the light-element line L101. In other words, if the current flowing through the light-element line L101 is I1, the value of the current I1 is 0 mA.
As described above, the value of the current Iall flowing to the LED driver circuit 100 is controlled so as to be a fixed value (40 mA). As a result, if the current flowing through the light-element line L102 is I2, the value of the current I1 is 40 mA. In other words, as shown in FIG. 6(a), if all of the plurality of light-element lines are operating normally, the current I1 and the current I2 are at 20 mA, but if any of the plurality of light-element lines is open, then a current at twice the value is concentrated in the remaining light-element lines.
Thus, in the general LED driver circuit 100, there are concerns such as the current concentrating in the light-element lines and the light-emitting elements being damaged due to the absolute maximum rating of the light-emitting elements included in the light-element lines being exceeded.
The configuration disclosed in Patent Document 1 is an example of a configuration to mitigate such concentration of current in the light-element lines.
FIG. 7 shows a configuration of the LED driver circuit 1000 disclosed in Patent Document 1. As shown in FIG. 7, in the LED driver circuit 1000, one or more light-element lines 1013 are connected to the output terminal of a fixed current power source supplying a fixed current, the light-element lines 1013 being connected in parallel to each other, and a biasing circuit 1014 is also connected in parallel, a fixed voltage source 1015 being connected to the biasing circuit 1014. The fixed voltage source 1015 causes a base potential of a transistor 1017 to be set at or above a prescribed value. Even if at least one of the LEDs 1016 of the light-element line 1013 became open due to damage or the like, it is possible to prevent a decrease in base potential of the transistors 1017 connected to other light-element lines 1013 using the fixed voltage source 1015, which allows the current at the other light-element lines 1013 to be fixed, and allows the LED driver circuit 1000 to be protected with a simple configuration.